Inspection of HDG

Inspection of HDG

Inspection of HDG

Inspecting Coating Thickness & Continuity

As a final step in the galvanizing process, the hot-dip galvanized coating is inspected for compliance with specifications. The coating thickness is the single most important component in determining a galvanized coatings quality. Coating thickness, however, is only one inspection criteria. Coating uniformity, adherence, and appearance should also be checked. Inspection of the galvanized product can be most effectively and efficiently conducted at the galvanizers plant where questions can be asked and answered quickly.There are a number of simple magnetic gauges used to give a convenient and reliable measurement of the zinc coating thickness, provided the instruments are properly calibrated. The three most common types of metal coating thickness gauges are:

Magnetic balance gauges, sometimes called banana gauges, measure variation in the force of attraction between two ferromagnetic bodies as a function of the distance between them. This type of gauge has the advantage of being able to measure the coating thickness in a horizontal or vertical position.

Pull-off magnetic gauges are also based on magnetic attraction to the underlying steel. These devices are shaped somewhat like a pen and are very convenient to make quick, rough estimates to determine whether the coating thickness meets specification.

Electronic gauges are the easiest and most accurate coating thickness measurement gauges available. They have the ability to connect to an assortment of probes, providing the ability to measure on any orientation.

One of the major advantages to specifying hot-dip galvanized steel is the ease of identifying coating defects immediately after galvanizing. Any areas that may remain uncoated are easily identifiable. If large areas see ASTM A 123) of the steel remain uncoated due to residues left on the steel from fabrication, then the steel must be stripped free of zinc and processed again. If small areas are left ungalvanized, they can be reconditioned using one of the three accepted methods of touch-up and repair, see ASTM A 780.Different coating appearance may result when steels of different chemistry or surface condition are galvanized. However, in most instances this is not a cause for rejection of the material, and does not affect the long-term corrosion protection. To fully understand how to inspect galvanized steel pieces, and what is acceptable and rejectable, take the Inspection Course.

Inspection of Hot-Dip Galvanized Steel

This course is intended to train individuals on the proper inspection techniques and requirements for hot-dip galvanized steel products. There are four sections in this course:

Hot-Dip Galvanizing Process

Galvanizing Standards

Types of Inspection

Repair

Upon completion of this course, you should be able to recognize specification requirements and perform all inspection steps to ensure conformance with the requirements. Additionally, any inspector who completes the course, and passes the test (80% or better) will receive a printable Certificate of Completion and will be listed on the AGA website as an inspector.

Disclaimer

The information contained in this course has been compiled by the American Galvanizers Association (AGA), a not-for-profit trade association whose members represent the after-fabrication hot-dip galvanizing industry throughout the United States, Canada, and Mexico. The AGA makes no endorsement and offers no evaluation of any vendors products, whether listed here or not.NextGalvanizing Process The term hot-dip galvanizing is defined as the process of immersing iron or steel in a bath of liquid zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. The coating is produced as the result of a metallurgical reaction between the liquid zinc and the iron in the steel. The coating forms an equal thickness on all surfaces immersed in the galvanizing kettle. This process, similar to the one seen in Figure 1, has been in use since 1742 and has provided long-lasting, maintenance-free corrosion protection at a reasonable cost for many years. The three main steps in the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment, each of which will be discussed in detail.

Figure 1: Model of the Hot-Dip Galvanizing ProcessSteel structures with visible evidence of corrosion are pictured in the series of photos in Figure 2. Rust and corrosion can be expensive for business owners and taxpayers because buildings, roads, and bridges, without sufficient corrosion protection, may need to be repaired often or even rebuilt.

The process is described in more detail later in this section. It is inherently simple, and this simplicity is a distinct advantage over other corrosion protection methods.

Figure 2: Corroding Steel StructuresPrev | NextSurface Preparation

Figure 3: Hanging of Steel ProductsThe first step in the hot-dip galvanizing process is intended to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. This is achieved by first hanging the steel using chains, wires, or specially designed dipping racks, as seen in Figure 3, to move the parts through the process. There are three cleaning steps to prepare the steel for galvanizing.Degreasing/Caustic Cleaning

First the steel is immersed in an acid degreasing bath or caustic solution in order to remove the dirt, oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water.

Pickling

Next the steel is immersed in an acid tank filled with either hydrochloric or sulfuric acid, as seen in Figure 4, which removes oxides and mill scale in a process called pickling. Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final stage of the surface preparation.

Figure 4: The Pickling TankFluxing

The purpose of the flux is to clean the steel of all oxidation developed since the pickling of the steel and to create a protective coating to prevent the steel from any oxidizing before entering the galvanizing kettle. One type of flux is contained in a separate tank, is slightly acidic, and contains a combination of zinc chloride and ammonium chloride. Another type of flux, top flux, floats on top of the liquid zinc in the galvanizing kettle, but serves the same purpose.

After being immersed in the degreasing, pickling, and fluxing tanks, the surface of the steel is completely free of any oxides or any other contaminants that might inhibit the reaction of the iron and liquid zinc in the galvanizing kettle.

Prev | NextGalvanizing

Figure 5: Hot-Dip Galvanizing KettleOnce the steel has been completely cleaned, it is ready for immersion in the liquid zinc. The galvanizing kettle contains zinc specified to ASTM B 6, a document that specifies any one of three different grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the zinc melt in order to promote certain desirable properties in the galvanized coating.

The galvanizing kettle, like the one seen in Figure 5, is typically operated at a temperature ranging from 820-860 F (438-460 C), at which point the zinc is in its liquid state. The steel products are immersed into the galvanizing kettle and remain in the kettle until the temperature of the steel has reached the temperature required to form a hot-dip galvanized coating. Once the interdiffusion reaction of iron and zinc is completed, the steel product is withdrawn from the zinc kettle. The entire dip usually lasts less than ten minutes, depending upon the thickness of the steel.

The coating, as seen in the micrograph in Figure 6, is typical for low silicon steels with silicon impurities less than 0.04% and where the thickness of the coating is limited by the interdiffusion of iron and zinc.

Figure 6: Photomicrograph of the galvanized coatingPost-Treatment

Filing Zinc DripsWhen the steel is removed from the galvanizing kettle, it may receive a post-treatment to enhance the galvanized coating. One of the most commonly used treatments is quenching. The quench tank contains mostly water but may also have chemicals added to create a passivation layer that protects the galvanized steel during storage and transportation. Other finishing steps include removal of zinc drips, or icicles, by grinding them off.

Prev | NextTime to First Maintenance

The estimated time to first maintenance for a hot-dip galvanized coating that experiences common atmospheric exposure can be seen in Figure 7. Time to first maintenance is defined as the time to 5% rusting of the substrate steel. The time to first maintenance of hot-dip galvanized steel is directly proportional to the zinc coating thickness.

Figure 7: Time to First Maintenance Chart for Hot-Dip Galvanized CoatingsPrev | Next

Other Corrosion Protection Systems

There are many other types of corrosion protection, such as coating steel with oil, grease, tar, asphalt, polymer coatings or paints, or corrosion protection materials such as stainless and weathering steel, sacrificial anodes, plating systems and impressed current systems. These are some of the most commonly used corrosion protection materials and systems and are sometimes used together with hot-dip galvanized steel. Most of these materials rely on barrier protection, while some of them rely on cathodic properties to prevent corrosion of the steel. The most effective type of corrosion protection that provides both barrier and cathodic protection is hot-dip galvanizing.There are also a wide variety of zinc coatings used for corrosion protection. Many people use galvanizing to describe all of these coatings, but each has its own unique characteristics and performance. These coatings have several applications based on their properties and respective thicknesses. The corrosion protection offered by a zinc coating is linearly related to its coating thickness. The most commonly used coatings are hot-dip galvanized, metallized, zinc-rich paint, galvannealed or galvanized sheet, and electroplated. The relative thickness for each of these zinc coatings can be seen in the photomicrograph (Figure 8). Below is a brief explanation of each type of zinc coating.

Figure 8: Photomicrograph of Zinc Coatings ThicknessesMetallizing

Metallizing is the general name for the technique of spraying a metal coating on the surface of non-metallic or metallic objects. This process is accomplished by feeding zinc in either wire or powder form into a heated gun, where it is melted and sprayed onto the surface to be coated using combustion gases and/or auxiliary compressed air to provide the necessary velocity. The limitations of this process include a difficulty in reaching recesses, cavities, and hollow spaces, even coating thickness and cost.Zinc-Rich Paint

Zinc-rich paint is applied to a clean, dry steel surface by either a brush or spray and usually contains an organic binder pre-mix. Paints containing zinc dust are classified as organic or inorganic, depending on the binder that they contain, and are discussed in more detail later in this course.Continuous Galvanizing

Figure 9: Continuous Galvanizing PlantThe continuous galvanizing process is a hot-dip process where a steel sheet, strip, or wire is cleaned, pickled, and fluxed on a processing line approximately 500 feet (154 m) in length, and running at speeds between 100 to 600 feet per minute (30 to 185 m per minute). In the coating of a steel sheet or strip, the galvanizing kettle contains a small amount of aluminum, which suppresses the formation of the zinc-iron alloys, resulting in a coating that is mostly pure zinc. A post-galvanizing, in-line heat treatment process known as galvannealing can also be used to produce a fully alloyed coating. Galvannealing is usually ordered by those wanting to paint over the zinc surface because the presence of alloy layers on the steel surface promotes paint adhesion. A photo of a continuous galvanizing plant is seen in Figure 9 and the common plant setup is shown in Figure 10.

Figure 10: Example of a Continuous ProcessElectroplating

The electroplating process, or zinc-plated coating, has a dull gray color, a matte finish, and a thin coating that ranges up to one mil (25 m) thick. This very thin coating restricts the use of zinc-plated products to indoor exposures. The specification ASTM B 633 lists the classes of zinc-plated steel coatings as Fe/Zn 5, Fe/Zn 8, Fe/Zn 12, and Fe/Zn 25, where Fe represents iron and Zn represents zinc, while the number indicates the coating thickness in microns. The main uses for this type of coating include screws, light switch plates, and other small products or fasteners.Prev | NextASTM Specifications

There are certain specifications that have been developed for hot-dip galvanizing in order to produce a high-quality coating. The most commonly used specifications design engineers and fabricators should become familiar with in order to promote a high-quality coating and ensure their steel design is suitable for hot-dip galvanizing are:

ASTM A 123/A 123M: Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products Single pieces of steel or fabrications with different types of steel products

ASTM A 153/A 153M: Standard Specification for Zinc Coating (Hot-Dip) on Iron and Hardware Fasteners and small products that are centrifuged after galvanizing to remove excess zinc

ASTM A 767/A 767M: Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement Reinforcing steel or rebar

ASTM A 780: Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings Touch-up procedures for coating bare spots on an existing hot-dip galvanized product

Other commonly used specifications in the hot-dip galvanizing industry include:

ASTM A 143/A 143M: Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement

ASTM A 384/A 384M: Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies

ASTM A 385/A 385M: Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)

ASTM B 6: Standard Specification for Zinc

ASTM D 6386: Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Paint

ASTM E 376: Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods

CAN/CSA G 164: Hot-Dip Galvanizing of Irregularly Shaped Articles

ISO 1461 Hot-Dip Galvanized Coatings on Fabricated Iron and Steel Assemblies Specifications and Test Methods

Lets examine a few of these specifications in more detail.

Prev | NextASTM A 123 for Structural Steel Products

Figure 11: Single Fabrication with Multiple Material CategoriesThe ASTM A 123/A 123M specification covers individual steel pieces as well as assemblies of various classes of material. The four material categories covered in ASTM A 123/A 123M include structural steel and plates, strips and bars, pipes and tubing, and wires. A fabrication can have more than one material category such as a frame assembly. Any combination of these products can be assembled into a single fabrication and then can be hot-dip galvanized, as seen in Figure 11.It is the responsibility of the designer and fabricator to ensure the product has been properly designed and built before the hot-dip galvanizing process. The galvanizer should be made aware of any necessary special instructions or requests in advance of shipping the materials to the galvanizing plant. These requests should be stated on the purchase order for the hot-dip galvanizing.

It is the responsibility of the galvanizer to ensure compliance with the specifications as long as the product has been designed and fabricated in accordance with the referenced specifications. However, if the galvanizer has to perform additional work in order to prepare the product for hot-dip galvanizing, such as drilling holes to facilitate drainage or venting, it must be approved by the customer. Once the material has been hot-dip galvanized, it can be fully inspected at the galvanizing plant prior to shipment.

Any materials rejected by the inspectors for reasons other than embrittlement may be stripped, regalvanized, and resubmitted for inspection. The ASTM specifications A 143/A 143M, ASTM A 384/A 384M, and ASTM A 385 provide guidelines for preparing products for hot-dip galvanizing. The requirements listed in ASTM A 123/A 123M include coating thickness, finish, appearance, and adherence. These are each defined below and discussed in more detail later in this course.ASTM A 123/A 123M Requirements

Coating Thickness / Weight dependent upon material category and steel thickness

Finish continuous, smooth, uniform

Appearance free from uncoated areas, blisters, flux deposits and gross dross inclusions as well as having no heavy zinc deposits that interfere with intended use

Adherence the entire coating should have a strong adherence throughout the service life of galvanized steel

The hot-dip galvanized coating is intended for products fabricated into their final shape that will be exposed to corrosive environmental conditions. Once a product has been hot-dip galvanized, any further fabrication, which very rarely occurs, may have negative effects on the corrosion protection of the coating. The coating grade is defined as the required thickness of the coating and is given in microns. All coating thickness requirements in specification ASTM A 123/A 123M, as seen in Tables 1 & 2, are minimums; there are no maximum coating thickness requirements in either specification.

Table 1: Minimum Average Coating Thickness Grade by Material Category (From ASTM A123)

Table 2: Coating Thickness Grade (From ASTM A 123)The time to first maintenance of hot-dip galvanized steel is directly proportional to the thickness of the hot-dip galvanized coating. With all other variables held constant, the thicker the zinc coating, the longer the life of the steel. The aim of the finish and appearance requirements is to ensure no coatings have problem areas that are deficient of zinc or have surface defects that would interfere with the intended use of the product. In addition, the coating should have a strong adherence throughout the service of the hot-dip galvanized steel.

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ASTM A 153 for Hardware

The specification ASTM A 153/A 153M applies to hardware products such as castings, fasteners, rolled, pressed and forged products, and miscellaneous threaded objects that will be centrifuged, spun, or otherwise handled to remove the zinc, as seen in Figure 12.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/fasteners2.jpg" \* MERGEFORMATINET Figure 12: Galvanized FastenersThe requirements for ASTM A 153/A 153M are very similar to those reported earlier for ASTM A 123/A 123M, except for the addition of threaded products and embrittlement requirements.

ASTM A 153/A 153M Requirements

Coating Thickness/Weight depends on the material category and steel thickness, values are listed in Table 3

Threaded Products areas with threads are not subject to the coating thickness requirement

Finish continuous, smooth, uniform

Embrittlement high tensile strength fasteners (>150ksi) and castings can be subject to embrittlement


Adherence the entire coating should have a strong adherence throughout the service life of hot-dip galvanized steel

There are fabrication steps that may impair the corrosion protection of the hot-dip galvanized coating, however, flaking or damage to the coating because of this is not case for rejection. In all cases, good steel selection results in the formation of a higher quality coating and finish on the product. The corrosion protection coating for threaded products is applied after the product has been fabricated and further fabrication may compromise the corrosion protection system. The one exception to this rule is the internal threads of a nut that should be over-tapped after the coating is applied in order to accommodate the coating thickness change on the thread of the bolts. In this case, the zinc on the bolt threads provides the corrosion protection to the uncoated threads in the nut.

There are certain fabrication techniques that can induce stresses into the steel and lead to brittle failure. There are precautions given in ASTM A 143/A 143M that should be taken in order to prevent embrittlement. In addition, selecting steels with appropriate chemistries can help prevent embrittlement of malleable castings. A reproduction and summary of the table given in ASTM A 153/A 153M, which is seen in Table 3, gives the different classes of products and the minimum coating thickness required by the specification.

Table 3: Minimum Average Coating Thickness by Material Class (From ASTM A 153)Prev | NextASTM A 767 for Reinforcing Steel

The specification ASTM A 767/A 767M is applicable exclusively to the hot-dip galvanizing of reinforcing steel, otherwise known as rebar, as seen in Figure 13, and is applicable to all types of rebar, both smooth and deformed. However, wire is not included.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/rebar2.jpg" \* MERGEFORMATINET Figure 13: Hot-Dip Galvanized RebarThe requirements in ASTM A 767/A 767M are also intended to produce a high quality zinc coating for corrosion protection.

ASTM A 767/A 767M Requirements

Identity the galvanizer is responsible for consistent material tracking if necessary

Coating Thickness/Weight material category and steel thickness

Chromating to prevent reaction between cement and recently galvanized material

Finish continuous, smooth, and uniform


Adherence should be tightly adherent throughout intended use of the product

Bend Diameters flaking and cracking due to fabrication after the hot-dip galvanizing process are not rejectable

Once rebar is delivered to be hot-dip galvanized, it is the galvanizers responsibility to track and maintain the identity of the product throughout the hot-dip galvanizing process until shipment of the finished product. Again, the analogous coating requirements in the areas of coating thickness, finish, and adherence are present in ASTM A 767/A 767M. However, this single product specification introduces a few new requirements that apply solely to hot-dip galvanized rebar. In ASTM A 767/A 767M, the coating requirement is given in mass of the zinc coating per surface area. A summary of the table given in ASTM A 767/A 767M and the minimum required coating thickness / weight of the classes is seen in Table 4.

Table 4: Mass of Zinc Coating (From ASTM A 767)This specification also introduces a new requirement to the galvanized coating known as chromating. Newly galvanized steel can react with wet cement and potentially form hydrogen gas as a product. As this evolved hydrogen gas travels through the concrete matrix toward the surface, voids can be created which weaken the bonding with the concrete or disturb the smoothness of the concrete surface. In order to help prevent and suppress this reaction, hot-dip galvanized rebar is dipped into a weak chromate quench solution after being removed from the galvanizing kettle.

The finish requirement for rebar is along the same lines as the finish requirements given in specifications ASTM A 123/A 123M and A 153/A 153M. The coating is intended for corrosion protection, so deficiencies that affect the coatings corrosion performance are grounds for rejection. In addition, since rebar is handled frequently during its installation, any tears or sharp spikes that make the material dangerous to handle are grounds for rejection.

Rebar is commonly bent prior to the hot-dip galvanizing process. The table below gives recommendations for bend diameters based upon the bare steel bar diameter before coating. Steel reinforcing bars that are bent cold prior to hot-dip galvanizing should be fabricated to a bend diameter equal to or greater than the specified values. However, steel reinforcing bars can be bent to diameters tighter than specified in Table 5 providing they are stress relieved at a temperature of 900 to 1050 F (480 to 560 C) for one hour per inch (25 mm) of diameter.

Table 5: Minimum Finished Bend Diameters (From ASTM A 767)Prev | NextOther Galvanizing Standards

There are Canadian and international specifications that could be used to specify hot-dip galvanizing on a project. The differences in these specifications and the ASTM specifications are minimal, and for the most part, only differ slightly in the minimum coating thickness/weight required for each type and thickness of product being hot-dip galvanized.

Other Specifications for Hot-Dip Galvanizing (Taken from CAN/CSA and ISO Standards)

CAN/CSA-G164 Hot Dip Galvanizing of Irregularly Shaped Articles

Scope

1. This standard specifies the requirements for zinc coating (galvanizing) by the hot-dipping process on iron and steel products made from rolled, pressed, or forged shapes such as structural sections, plates, bars, pipes, or sheets 1 mm thick or thicker.

2. Applies to both unfabricated and fabricated products such as assembled steel products, structural steel fabrications, large hollow sections bent or welded before galvanizing, and wire work fabricated from uncoated steel wire.

3. Applies to steel forgings and iron castings that are to be galvanized separately or in batches.

4. Does not apply to continuous galvanizing of chain link fence fabric, wire, sheet, and strip.

5. Does not apply to pipe and conduit that are normally hot dip galvanized by a continuous or semi continuous automatic process.

6. The values stated in SI units are to be regarded as the standard. The values in parentheses are imperial units and are included for information only.

ISO 1461 Hot Dip Galvanized Coatings on Fabricated Iron and Steel Articles

Scope: This Standard specifies the general properties of and methods of test for coatings applied by hot dipping in zinc (containing not more than 2% of other metals) on fabricated iron and steel articles.

It does not apply to:

1. Sheet and wire continuously hot dip galvanized;

2. Tube and pipe hot dip galvanized in automatic process;

3. Hot dip galvanizing products for which specific standards exist and which may include additional requirements or requirements different from those of this European Standard.

4. After-treatment/over coating of hot dip galvanized articles is not covered by this standard.

NOTE Individual product standards can incorporate this standard for the coating by quoting its number, or may incorporate it with modifications specific to the product.

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Types of Inspection

In this section, the type of inspections performed on hot-dip galvanized steel will be discussed. The various inspections are used to verify the necessary specifications for the galvanized product are met. These techniques for each test method are specified in ASTM A 123/A 123M, A 153/A 153M, or A 767/A 767M, depending upon the type of product being inspected. The most common inspections, listed below, range from a simple visual inspection to more sophisticated tests to determine embrittlement or adhesion.

Coating Thickness magnetic gauges, optical microscopy

Coating Weight weigh-galvanize-weigh, and weigh-strip-weigh

Finish and Appearance visual inspection

Additional Tests

Adherence stout knife

Embrittlement similar bend radius, sharp blow, and steel angle

Chromating spot test

Bending minimum finished bend diameter table

Sampling

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Coating Thickness

The term coating thickness refers to the thickness of zinc applied to steel, while coating weight refers to the amount of zinc applied to steel for a given surface area. Two different methods are used in order to measure the coating thickness of hot-dip galvanized steel.

Figure 14: Pencil-Style GaugeThe first method to measure coating thickness involves using magnetic thickness gauges. There are three different types of magnetic thickness gauges and all can be used quite easily in the galvanizing plant or in the field.

The first type of magnetic thickness gauge is very small and utilizes a spring-loaded magnet encased in a pencil-like container, as seen in Figure 14. The tip of the gauge is placed on the surface of the steel and is slowly pulled off in a continuous motion. When the tip of the gauge is pulled away from the surface of the steel, the magnet, near the tip, is attracted to the steel. A graduated scale indicates the coating thickness at the instant immediately prior to pulling the magnet off the surface of the steel. The accuracy of this gauge requires it to be used in the true vertical plane because, due to gravity, there is more error associated with measurements taken in the horizontal plane or overhead positions. The measurement should be made multiple times because the absolute accuracy of this type of gauge is below average and it is difficult to determine the true coating thickness when only one reading is taken.

Figure 15: Banana GaugeA banana gauge, as seen in Figure 15 is the second type of thickness gauge. With this gauge, coating thickness measurements are taken by placing the rubber magnet housing on the surface of the product with the gauge held parallel to the surface. A scale ring is rotated clockwise to bring the tip of the instrument in contact with the coated surface and rotated counter-clockwise until a break in contact can be heard and felt. The position of the scale ring when the magnetic tip breaks from the coated surface displays the coating thickness. This type of gauge has the advantage of being able to measure coating thickness in any position, without recalibration or interference from gravity.

Figure 16: Electronic/Digital Thickness GaugeThe electronic or digital thickness gauge, as seen in Figure 16 is the most accurate and arguably, the easiest thickness gauge to operate. The electronic thickness gauge is operated by simply placing the magnetic probe onto the coated surface and then a digital readout displays the coating thickness. Electronic gauges have the advantage of not requiring recalibration with probe orientation, but do require calibration with shims of different thicknesses in order to verify the accuracy of the gauge at the time it is being used. These shims are measured and the gauge is calibrated according to the thickness of the shim, and then this process is repeated for shims of different thicknesses until the gauge is producing an accurate reading in all ranges of thickness.ASTM E 376

The specification ASTM E 376 contains information for measuring coating thickness using magnet or electromagnetic current. It also provides some tips for obtaining measurements with the greatest accuracy, as well as describing how the physical properties, the structure, and the coating can interfere with the measurement methods. The requirements for ASTM E 376, as seen below, are intended to make the coating thickness measurements using magnet or electromagnetic current as accurate as possible.

ASTM E 376 Requirements Measurements on large products should be made at least four inches from the edge to avoid edge effects

Measurement readings should be as widely dispersed as possible

There are some general guidelines, as seen below, for reducing error and ensuring the most accurate readings are being collected when using magnetic thickness gauge instruments.

Guidelines for Reducing Error Recalibrate frequently, using non-magnetic film standards or shims above and below the expected thickness value

Readings should not be taken near an edge, a hole, or inside corner

Readings taken on curved surfaces should be avoided if possible

Test points should be on regular areas of the coating

Take at least five readings to obtain a good, true value which is representative of the whole sample

Figure 17: Optical MicroscopyThe second method used to measure the coating thickness involves optical microscopy, as seen in Figure 17. This is a destructive technique and is typically only used for inspection of the coating of single specimen samples that have failed magnetic thickness readings or for research studies. Since it is not a common method, the accuracy is highly dependent on the expertise of the operator.

Prev | NextCoating Weight

The term coating weight refers to the amount of zinc applied to a product for a given surface area. Two different methods can be used to measure the coating weight of hot-dip galvanized steel.

The first method to measure the coating weight involves using a process called weigh-galvanize-weigh, and is only appropriate for single specimen samples. The zinc coating weight from this technique is underestimated because the actual coating is made up of both iron and zinc and this method will only measure the added zinc weight in the coating. In addition, it can be very difficult to measure and calculate the surface area of a complex steel fabrication, and this makes coating weight values even less accurate.

Weigh-strip-weigh is the second method used to measure coating weight, and again is only appropriate for single specimen samples. This method is destructive since it removes the hot-dip galvanized coating during the measurement. This process involves first weighing the specimen, stripping it of all zinc coating that was added, and then weighing it again. The difference in the weights is then equal to the amount of coating added during the galvanizing process. However, this method is usually only used on very small products like nails, and can be inaccurate because when the coating is stripped there may be some base metal stripped along with the coating. This means that there may be extra iron included in the weight measurement, making for a higher than actual zinc coating weight.Prev | NextFinish & Appearance

The inspection of finish and appearance is done with an unmagnified visual inspection. This inspection is performed by fully observing all parts and pieces of a hot-dip galvanized product to ensure all necessary components and specifications have been met. It is done in order to observe surface conditions, both inside and out, and check all contact points, as well as welds, junctions, and bend areas.

Appearance

The appearance of the hot-dip galvanized coating can vary from piece to piece, and even section to section of the same piece. There are a number of reasons for the non-uniform appearance, but it is important to note appearance has no bearing on the corrosion protection of the galvanized piece. This section will overview the reasons for differences in appearance.

Finish

This section will review a number of possible surface defects visible on the galvanized coating. Some of these surface defects are rejectable, as they will seriously lower the corrosion protection, while others have little or no effect on the corrosion performance and are acceptable.Prev | NextDifferent Appearances

The appearance of hot-dip galvanized steel immediately after galvanizing can be bright and shiny, spangled, matte gray, or a combination of these. There are a number of reasons for the difference in appearance, as explored here, but regardless if the piece is shiny or dull, the appearance has no effect on the corrosion performance. And in time after exposure to the environment, all galvanized coatings will take on a uniform matte gray appearance.

Reasons for Different Appearances

Steel Chemistry

The most common reason for galvanized steel to have different appearances is the chemistry of the steel pieces. There are two elements of steel chemistry which most strongly influence the final appearance; silicon and phosphorous. Both silicon and phosphorous promote coating growth, and this thicker coating is responsible for the differing appearance.

The amount of silicon added during the steel making process to deoxidize the steel can create differences in appearance of galvanized products. The recommended silicon composition is either less than 0.04% or between 0.15% and 0.25%. Any steels not within these ranges are considered reactive steels and are expected to form zinc coatings that tend to be thicker.

In addition to producing thicker coatings, highly reactive steels tend to have a matte gray or mottled appearance instead of the typical bright coating. This difference in appearance is a result of the rapid zinc-iron intermetallic growth that consumes all of the bright, pure zinc. This growth of the intermetallic layer is generally out of the galvanizers control, because they usually do not have prior knowledge of the steels composition. However, this increased coating thickness can be beneficial in some respects because time to first maintenance is directly proportional to coating thickness.

In Figure 18, the micrograph on the left shows a regular zinc-iron alloy, while the micrograph on the right shows an irregular zinc-iron alloy. These clearly show the microscopic level differences that can occur due to the amount of silicon in the steel being hot-dip galvanized.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/irregular.jpg" \* MERGEFORMATINET Figure 18: Regular vs. Irregular Zinc-Iron Alloy LayersThe Sandelin curve, as seen in Figure 19, compares the zinc coating thickness to the mass percentage of silicon in the steel. The area on the graph labeled I is called the Sandelin area and the coatings tend to be thick and dull gray as a direct result of the percentage of silicon present in the base steel. This area is known as the Sandelin range since Dr. Sandelin, a metallurgist, performed the experimental work to show this behavior of galvanized steel. The Sandelin area is roughly between 0.05% and 0.15% silicon. The area on the graph labeled II, which represents a steel content of greater than 0.25% silicon, shows the coating thickness increases with increased silicon content and then starts to level off at around 0.4% silicon.

Figure 19: Sandelin Curve

Figure 20: Coating Due to PhosphorousIn addition to silicon, the presence of phosphorus influences the reaction between the liquid zinc and the steel, as seen in Figure 20. Phosphorus is generally considered an impurity in steel except where its beneficial effects on machinability and resistance to atmospheric corrosion are desired. Some steels such as ASTM A 242 Type 1 present problems because they may contain both a high level of phosphorus and a high level of silicon. The presence of phosphorus generally produces smooth dull coating areas and ridges of a thicker coating where there is increased intermetallic growth. The end-result is a rough surface with ridges appearance.

Figure 21 is an example of products with separate galvanized pieces that have very different appearances due to the difference in steel chemistry. However, all of these products still have an equal amount of corrosion resistance throughout and are acceptable.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/dullshiny2.jpg" \* MERGEFORMATINET Figure 21: Shiny vs. DullCooling Rate

Figure 22: Coating Appearance Due to Cooling Rate DifferenceA visually dull or shiny coating on a product can be caused by the different rate of cooling of a product. In Figure 22, the outer edges were cooled rapidly, which allowed free zinc or an eta layer to form on top of the intermetallic layers. The zinc in the center of the product that would have formed the eta layer was consumed in the reaction with the iron after the part was removed from the galvanizing kettle and formed an intermetallic layer that gives the dull gray look. Eventually as the product weathers, the differences in appearance will disappear and it will become a dull gray color throughout.

Steel Processing

Figure 23: Coating Appearance Due to Steel ProcessingIn addition to temperature and chemistry of the steel, the processing of the steel can also create a bright or dull appearance in galvanized products. The top rail in Figure 23 has a winding pattern of dull gray areas corresponding to processing during the tube making. The stresses in the steel affect the intermetallic formation and can create this striped look. The corrosion protection is not affected and these parts are acceptable.

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Finish: Visual Defects

As stated before, the hot-dip galvanized coating could have any number of surface defects. This section will review the various defects and discuss whether or not they are cause for rejection according to the specification. The surface defects reviewed are:

A C Bare Spots

Blasting Damage

Chain and Wire Marks

Clogged Holes

Clogged Threads

D E Delamination

Distortion

Drainage Spikes

Dross Inclusions

Excess Aluminum in Galvanizing Bath

F O Fish Boning

Flaking

Flux Inclusions

Oxide Lines

P R

Products in Contact

Rough Surface Condition

Runs

Rust Bleeding

S T

Sand Embedded in Casting

Striations

Steel Surface Condition

Surface Contaminant

Touch Marks

U Z

Weeping Weld

Welding Blowouts

Welding Spatter

Wet Storage Stain

Zinc Skimmings

Zinc Splatter

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Visual Defects: A-C

Bare Spots

Bare spots, defined as uncoated areas on the steel surface, are the most common surface defect and occur because of inadequate surface preparation, welding slag, sand embedded in castings, excess aluminum in the galvanizing kettle, or lifting aids that prevent the coating from forming in a small area. Only very small areas, less than 1 inch in the narrowest dimension with a total of no more than 0.5% of the accessible surface area, may be renovated using ASTM A 780. This means narrow, bare areas may be repaired; however, if they are greater than one inch-square areas, the product must be regalvanized. In order to avoid bare spots, like those seen in Figure 24, the galvanizer must ensure the surfaces are clean and no contaminants are present after pretreatment. If the size of the bare spot or total surface area causes rejection, the parts may be stripped, regalvanized, and then re-inspected for compliance to the standards and specifications.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/barespot2.jpg" \* MERGEFORMATINET Figure 24: Bare Spots

Figure 25: Blasting DamageBlasting Damage

Blasting damage creates blistered or flaking areas on the surface of the galvanized product. Blasting damage follows abrasive blasting prior to painting of the galvanized steel. It is caused by incorrect blasting procedures creating shattering and delamination of the alloy layers in the zinc coating. Blasting damage, as seen in Figure 25, can be avoided when careful attention is paid to preparation of the product for painting. In addition, blast pressure should be greatly reduced according to ASTM D 6386. Since blasting damage is induced by a post-galvanizing process, the galvanizer is not responsible for the damage.

Figure 26: Chain and Wire MarksChain and Wire Marks

Another type of surface defect occurs when steel is lifted and transported around the galvanizing plant using a chain or wire. These lifting aids can leave uncoated areas on the finished product that will need to be repaired. The superficial marks, like those seen in Figure 26, left on the galvanized coating from the lifting attachments are not grounds for rejection as long as marks can be repaired. ASTM specifications do not allow any bare spots on the finished galvanized part.

Figure 27: Clogged HolesClogged Holes

Clogged holes are holes partially or completely clogged with zinc metal. A good example is the screen shown in Figure 27. The zinc was trapped because liquid zinc will not drain easily from holes less than 3/10 (8mm) in diameter due to its high surface tension. Clogged holes can be minimized by making all holes as large as possible. The trapped zinc can be removed by using active fettling when the part is in the galvanizing kettle, vibrating the cranes to jostle the parts, or blowing compressed air onto the galvanized products. This condition is not a cause for rejection, unless it prevents the part from being used for its intended purpose.

Figure 28: Clogged ThreadsClogged Threads

Clogged threads are caused by poor drainage of a threaded section after the product is withdrawn from the galvanizing kettle. These clogged threads, as seen in Figure 28, can be cleaned by using post-galvanizing cleaning operations such as a centrifuge or by heating them with a torch to about 500 F (260 C) and then brushing them off with a wire brush to remove the excess zinc. Clogged threads must be cleaned before the part can be accepted.

Prev | NextVisual Defects: D-E

Figure 29: DelaminationDelamination

Delamination or peeling creates a rough coating on the steel where the zinc has peeled off. There are a number of causes for zinc peeling. Many large galvanized parts take a long time to cool in the air and form zinc-iron layers after they have been removed from the galvanizing kettle. This continued coating formation leaves behind a void between the top two layers of the galvanized coating. If there are many voids formed, the top layer of zinc can separate from the rest of the coating and peel off the part. If the remaining coating still meets the minimum specification requirements, then the part is still acceptable. If the coating does not meet the minimum specification requirements then the part must be rejected and regalvanized. If delamination, as seen in Figure 29, occurs as a result of fabrication after galvanizing, such as blasting before painting, then the galvanizer is not responsible for the defect.

Figure 30: DistortionDistortion

Distortion, as seen in Figure 30, is defined as the buckling of a thin, flat steel plate or other flat material such as wire mesh. The cause of this is differential thermal expansion and contraction rates for the thin, flat plate and mesh than the thicker steel of the surrounding frame. In order to avoid distortion, use a thicker plate, ribs, or corrugations to stiffen flat sections or make the entire assembly out of the same thickness steel. Distortion is acceptable, unless distortion changes the part so that it is no longer suitable for its intended use.

Figure 31: Drainage SpikesDrainage Spikes

Drainage spikes or drips are spikes or tear drops of zinc along the bottom edges of the product. These result when the surfaces of the product are processed horizontal to the galvanizing kettle, preventing proper drainage of the zinc from the surface as the product is withdrawn from the kettle. Drainage spikes, as seen in Figure 31, are typically removed during the inspection stage by a buffing or grinding process. Drainage spikes or drips are excess zinc and will not affect corrosion protection, but are potentially dangerous for anyone who handles the parts. These defects must be removed before the part can be accepted.

Dross Inclusions

Dross inclusions are a distinct zinc-iron intermetallic alloy that becomes entrapped or entrained in the zinc coating. This is caused by picking up zinc-iron particles from the bottom of the kettle. Dross, as seen in Figure 32, may be avoided by changing the lifting orientation or redesigning the product to allow for proper drainage. If the dross particles are small and completely covered by zinc metal, they will not affect the corrosion protection and are acceptable. If the dross particles are large, then the dross must be removed and the area repaired.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/dross.jpg" \* MERGEFORMATINET Figure 32: Dross Inclusions

Figure 33: Excess Aluminum in Galvanizing BathExcess Aluminum in Galvanizing Bath

Another type of surface defect, shown in Figure 33, is caused by an excess amount of aluminum in the galvanizing bath. This creates bare spots and black marks on the surface of the steel. The excess aluminum can be avoided by ensuring proper control of the aluminum level in the galvanizing bath by means of regular sampling and analysis, and by adjusting the levels in a regular and controlled manner. For small areas of bare spots, the part may be repaired as detailed in the specification. If this condition occurs over the entire part, then it must be rejected and regalvanized.

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Visual Defects: F-O

Figure 34: Fish BoningFish Boning

Fish boning is an irregular pattern over the entire surface of the steel part. This is caused by differences in the surface chemistry of a large diameter steel piece and variations in the reaction rate between the steel and zinc. These reaction differences cause the thickness of the galvanized coating to vary in sharply defined zones across the surface. Fish boning, as seen in Figure 34, has no effect on the corrosion protection provided by the zinc coating and is not cause for rejection of the hot-dip galvanized part.

Figure 35: Micrograph of FlakingFlaking

Flaking results when heavy coatings develop in the galvanizing process, usually 12 mils or greater. This generates high stresses at the interface of the steel and the galvanized coating and causes the zinc to become flaky and separate from the surface of the steel. Flaking can be avoided by minimizing the immersion time in the galvanizing kettle and cooling of the galvanized steel parts as quickly as possible. Figure 35 shows a micrograph of flaking. In addition, using a different steel grade, if possible, may also help avoid flaking. If the area of flaking is small, it can be repaired and the part can be accepted; however, if the area of flaking is larger than allowed by the specifications, the part must be rejected and regalvanized.

Figure 36: Flux InclusionFlux Inclusions

Flux inclusion can be created by the failure of the flux to release during the hot-dip galvanizing process. If this occurs, the galvanized coating will not form under this flux spot. If the area is small enough, it must be cleaned and repaired; otherwise, the part must be rejected. Flux spots can increase if the flux is applied using the wet galvanizing method, which is when the flux floats on the zinc bath surface. Flux deposits on the interior of a hollow part, such as a pipe or tube, as seen in Figure 36, cannot be repaired, thus the part must be rejected. Any flux spots or deposits, picked up during withdrawal from the galvanizing kettle do not warrant rejection if the underlying coating is not harmed, and the flux is properly removed.

Figure 37: Oxide LinesOxide Lines

Oxide lines are light colored oxide film lines on the galvanized steel surface. Oxide lines are caused when the product is not removed from the galvanizing kettle at a constant rate. This may be due to the shape of the product or the drainage conditions. Oxide lines, as seen in Figure 37, will fade over time as the entire zinc surface oxidizes. They will have no effect on the corrosion performance; only the initial appearance will be affected. This condition is not a cause for rejection of the hot-dip galvanized parts.

Prev | NextVisual Defects: P-R

Figure 38: Products in ContactProducts in Contact

Another type of surface defect is caused by products that come in contact with each other or are stuck together. This usually occurs when many small products are hung on the same fixture, which creates the chance products may become connected or overlapped during the galvanizing process, as seen in Figure 38. The galvanizer is responsible for proper handling of all products in order to avoid this defect. In addition, if the surface of a product has a larger bare area than the specified repair requirement allows, then that product must be rejected and regalvanized.

Rough Surface Condition

Rough surface condition or appearance is a uniformly rough coating with a textured appearance over the entire product. The cause for this rough surface condition is hot-rolled steel with a high level of silicon content. This can be avoided by purchasing steel with a silicon content less than 0.03% of the steel by weight. Rough surface condition, as seen in Figure 39, can actually have a positive effect on corrosion performance because of the thicker zinc coating produced. One of the few situations where rough coating is cause for rejection is if it occurs on handrails. The corrosion performance of galvanized steel with rough coatings is not affected by the surface roughness.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/roughsurface.jpg" \* MERGEFORMATINET Figure 39: Rough Surface Condition

Figure 40: RunsRuns

Runs are localized thick areas of zinc on the surface. Runs occur when zinc freezes on the surface of the product during removal from the zinc bath. This is more likely to occur on thinner sections with large surface areas that cool quickly. In order to avoid runs, as seen in Figure 40, adjustments of the dipping angles can be made, if possible, to alter the drainage pattern to a more acceptable mode. If runs are unavoidable and will interfere with the intended application, they can be buffed. Runs are not cause for rejection.

Rust Bleeding

Rust bleeding appears as a brown or red stain that leaks from unsealed joints after the product has been hot-dip galvanized. It is caused by pre-treatment chemicals that penetrate an unsealed joint. During galvanizing of the product, moisture boils off the trapped treatment chemicals leaving anhydrous crystal residues in the joint. Over time, these crystal residues absorb water from the atmosphere and attack the steel on both surfaces of the joint, creating rust that seeps out of the joint. Rust bleeding, as seen in Figure 41, can be avoided by seal welding the joint where possible or by leaving a gap greater than 3/32 (2.4mm) wide in order to allow solutions to escape and zinc to penetrate during hot-dip galvanizing. If bleeding occurs, it can be cleaned up by washing the joint after the crystals are hydrolyzed. Bleeding from unsealed joints is not the responsibility of the galvanizers and is not cause for rejection.

INCLUDEPICTURE "http://www.galvanizeit.org/images/uploads/contentPhotos/insepctionCourse/rustbleeding2.jpg" \* MERGEFORMATINET Figure 41: Rust BleedingPrev | Next

Visual Defects: S-T

Figure 42: Sand Embedded in CastingSand Embedded in Casting

Another type of surface defect occurs when sand becomes embedded in the castings and creates rough or bare spots on the surface of the galvanized steel. Sand inclusions are not removed by conventional acid pickling, so abrasive cleaning should be done at the foundry before the products are sent to the galvanizer. This type of defect also leaves bare spots and must be cleaned and repaired or the part must be rejected, stripped, and regalvanized. Sand embedded in a casting can be seen in Figure 42.

Figure 43: StriationsStriations

Striations are characterized by raised parallel ridges in the galvanized coating, mostly in the longitudinal direction. This can be caused when sections of the steel surface are more highly reactive then the areas around them. These sections are usually associated with segregation of steel impurities, especially phosphorous, created during the rolling process in steel making. Striations, as seen in Figure 43, are related to the type of steel galvanized and while the appearance is affected, the performance of the corrosion protection is not. Striations are acceptable on most parts; however, if the striations happen to occur on handrails, then the parts must be rejected and regalvanized. Sometimes regalvanizing does not improve the striations and the handrail must be refabricated out of better quality steel.

Figure 44: Surface ContaminentsSurface Contaminant

When surface contaminants create an ungalvanized area where the contaminant was originally applied, a surface defect may occur. This is caused by paint, oil, wax, or lacquer not removed during the pretreatment cleaning steps. Surface contaminants, as seen in Figure 44, should be mechanically removed prior to the galvanizing process. If they result in bare areas, then the repair requirements apply and small areas may be repaired, but a large area is grounds for rejection and the entire part must be regalvanized.

Figure 45: Touch MarksTouch Marks

Another type of surface defect is known as touch marks, which are damaged or uncoated areas on the surface of the product. Touch marks are caused by galvanized products resting on each other or by the material handling equipment used during the galvanizing operation. Touch marks, as seen in Figure 45, are not cause for rejection if they meet the size criteria for repairable areas. They must be repaired before the part is accepted.

Prev | NextVisual Defects: U-Z

Figure 46: Weeping WeldWeeping Weld

Weeping welds stain the zinc surface at the welded connections on the steel. They are caused by entrapped cleaning solutions that penetrate the incomplete weld. In order to avoid weeping welds for small overlapping surfaces, completely seal weld the edges of the overlapping area. For larger overlapping areas, the area cannot be seal welded since the volume expansion of air in the trapped area can cause explosions in the galvanizing kettle. To avoid weeping welds in large overlapping areas, the best plan is to provide a 3/32 (2.4mm) or larger gap between the two pieces when welding them and let the zinc fill the gap between the pieces. This will actually make a stronger joint when the process is complete. Weeping welds, as seen in Figure 46, are not the responsibility of the galvanizer and are not cause for rejection.

Figure 47: Welding BlowoutsWelding Blowouts

Welding blowout is a bare spot around a weld or overlapping surface hole. These are caused by pre-treatment liquids penetrating the sealed and overlapped areas that boil out during immersion in the liquid zinc. This causes localized surface contamination and prevents the galvanized coating from forming. In order to avoid welding blowouts, as seen in Figure 47, check weld areas for complete welds to insure there is no fluid penetration. In addition, products can be preheated prior to immersion into the galvanizing kettle in order to dry out overlap areas as much as possible. Welding blowouts cause bare areas that must be repaired before the part is acceptable.

Figure 48: Welding SpatterWelding Spatter

Welding spatter appears as lumps in the galvanized coating adjacent to weld areas. It is created when welding spatter is left on the surface of the part before it is hot-dip galvanized. In order to avoid welding spatter, welding residues should be removed prior to hot-dip galvanizing. Welding spatter, as seen in Figure 48, appears to be covered by the zinc coating, but the coating does not adhere well and can be easily removed. This type of defect can leave an uncoated area or bare spot if the zinc coating is damaged and must be cleaned and properly repaired.

Wet Storage Stain

Wet storage stain is a white, powdery surface deposit on freshly galvanized surfaces. It is caused by newly galvanized surfaces being exposed to fresh water, such as rain, dew, or condensation that react with the zinc metal on the surface to form zinc oxide and zinc hydroxide. It is found most often on tightly stacked and bundled items, such as galvanized sheets, plates, angles, bars, and pipes. Wet storage stain can have the appearance of light, medium, or heavy white powder on the galvanized steel product. Each of these appearances can be seen from right to left in Figure 49.

One method to avoid wet storage stains is to passivate the product after galvanizing by using a chromate quench solution. Another precaution is to avoid stacking products in poorly ventilated, damp conditions. Light or medium wet storage stain will weather over time in service and is acceptable. In most cases, wet storage stain does not indicate serious degradation of the zinc coating, nor does it necessarily imply any likely reduction in the expected life of the product. However, heavy wet storage stain should be removed mechanically or with appropriate chemical treatments before the galvanized part is put into service. Heavy storage stain must be removed or the part must be rejected and regalvanized.

Figure 49: Wet Storage Stain

Figure 50: Zinc Skimming InclusionsZinc Skimmings

Skimming deposits are usually caused when there is no access to remove the skimmings during the withdrawal of the steel from the galvanizing kettle. The skimmings on the liquid zinc surface are trapped on the zinc coating. In order to remove zinc skimmings without harming the soft zinc coating underneath, lightly brush them off the surface of the galvanized steel during the in-house inspection stage with a nylon-bristle brush. Zinc skimmings, as seen in Figure 50, are not grounds for rejection. The zinc coating underneath is not harmed during their removal and it meets the necessary specifications.

Figure 51: Zinc SplatterZinc Splatter

Zinc splatter is defined as splashes and flakes of zinc that loosely adhere to the galvanized coating surface. Zinc splatter is created when moisture on the surface of the galvanizing kettle causes liquid zinc to pop and splash droplets onto the product. These splashes create flakes of zinc loosely adherent to the galvanized surface. Zinc splatter, as seen in Figure 51, will not affect the corrosion performance of the zinc coating and is not cause for rejection. The splatter does not need to be cleaned off the zinc coating surface, but can be if a consistent, smooth coating is required.

Prev | NextAdditional Tests

Adherence Test

Figure 52: Stout Knife TestTesting of the zinc coating adherence to the steel is achieved using a stout knife. The steps used in this test are listed below and a photo of the test being performed can be seen in Figure 52. The coating shall be deemed not adherent if it flakes off and exposes the base metal in advance of the knifepoint. The test is not an attempt to pare or whittle the zinc coating. If the coating is adherent the knife should put a slight mark in the zinc metal surface, but should not cause any delamination of the coating layers.

Adhesion Test with a Stout Knife Push down point of stout knife

Coating must not flake off exposing the base metal

Do not perform at edges or corners of the product

No paring or whittling with knife is acceptable

Bending Test

The hot-dip galvanized coating on a steel bar must withstand bending without flaking or peeling when the bending test is preformed in accordance with the specifications in ASTM A 143. There are various tests used to assess the ductility of steel when subjected to bending. One test may include the determination of the minimum radius or diameter required to make a satisfactory bend. Another test may include the number of repeated bends that the material can withstand without failure when it is bent through a given angle and over a definite radius.

Rebar is commonly bent prior to the hot-dip galvanizing process. Steel reinforcing bars bent cold prior to hot-dip galvanizing should be fabricated to a bend diameter equal to or greater than the specified value in ASTM A 767/A 767M. However, steel reinforcing bars can be bent to diameters tighter than the specified values if they are stress relieved at a temperature of 900 to 1050 F (480 to 560 C) for one hour per inch (25mm) of diameter. Chromating Test

The specification to determine the presence of chromate on zinc surfaces is ASTM B 201. This test involves placing drops of a lead acetate solution on the surface of the product, waiting 5 seconds, and then blotting it gently. If this solution creates a dark deposit or black stain, then there is unpassivated zinc present. A clear result indicates the presence of a chromate passivation coating.

Embrittlement Test

When there is suspicion of potential embrittlement of a product, it may be necessary to test a small group of the products to measure the ductility. These tests are usually destructive to the zinc coating and possibly to the product as well. Products suspected of embrittlement shall be tested according to the specification ASTM A 143. Depending on the service conditions the product will be exposed to, one of three embrittlement tests may need to be performed. These embrittlement tests include the similar bend radius test, sharp blow test, and steel angle test. The embrittlement test uses a known force to provide a stress that should be lower than the yield stress of the part. If there is a fracture or permanent damage created during the testing process, the parts must be rejected.

Prev | NextSampling

A sampling protocol has been developed by ASTM to ensure high quality products because the inspection of the coating thickness for every piece of material galvanized in a project would not be practical. ASTM A 123/A 123M states for a unit of products whose surface area is equal to or less than 160 in (1032 cm), the entire surface of each test product constitutes a specimen. In the case of a product containing more than one material category or steel thickness range, that product will contain more than one specimen. In addition, products with surface areas greater than 160 in (1032 cm) are multi-specimen products. There are four important terms used in the ASTM specifications and each is defined below.

Sampling Terms

Lot unit of production or shipment from which a sample is taken for testing

Sample a collection of individual units of product from a single lot

Specimen the surface of an individual test product or a portion of a test product which is a member of a lot or a member of a sample representing that lot

Test Product an individual unit of product that is a member of the sample

For single specimen products, each randomly selected product is a specimen. In thickness measurement tests, five measurements are taken widely dispersed over the surface area of the specimen in order to represent the total coating thickness. The mean value of the five coating thicknesses for one specimen must have a minimum average coating thickness grade of not less than one grade below the minimum average coating thickness for the material category. In Figure 53, the separation of a lot into a sample and individual specimen is shown.

Figure 53: Single Specimen Product SamplingA multi-specimen product is defined as having a surface area that may be larger than 160 in (1032 cm), have multiple steel thicknesses, or contain more than one coating category. In order to test coating thickness of products whose surface area is greater than 160 in (1032 cm), they are subdivided into three continuous local sections with equivalent surface areas, each of which constitutes a unique specimen. In the case of any such local section containing more than one material category or steel thickness range, that section will contain more than one specimen. In Figure 54, the separation of a lot into a sample and individual specimen is shown.

Figure 54: Mutli-Specimen Product SamplingFor products hot-dip galvanized to either ASTM A 123/A 123M or A 153/A 153M, Table 6 is used to determine the minimum number of specimens for sampling from a given lot size.

No. of Pieces in LotNo. of Specimens

3 or lessAll

4 to 5003

501 to 12005

1201 to 32008

3201 to 10,00013

10,001+20

Table 6: Minimum Number of Specimensfor ASTM A 123 and A 152

For rebar hot-dip galvanized according to ASTM A 767, the information below is used to determine the minimum number of samples per lot, measurements per sample, and the total number of measurements required for each of the different coating thickness measurement techniques.

Magnetic Thickness:

3 samples per lot

5 or more measurements per sample

15 measurements, at the minimum, comprise the average

Microscopy Method:

5 samples per lot

4 measurements per sample

20 measurements, at minimum, comprise the average

Stripping and Weighing:

3 samples per lot

The minimum average coating thickness for a lot is the average of the specimen values and must meet the minimum for the material category. The minimum for an individual specimen is one grade below the minimum for the material category. An individual measurement has no minimum, but bare areas are not allowed on the part. The final inspection of a part shall include thickness measurements and visual inspection. All parts that do not meet the requirement must be resorted and reinspected or rejected and then regalvanized.Prev | NextRepair

If the galvanized product does not meet all of the requirements of the specification, it must be repaired or rejected along with the lot it represents. When repair of the product is allowed by the specification or bare spots are present, the galvanizer is responsible for the repair unless directed otherwise by the purchaser. The specifications allow for some retesting of products that represent lots or retesting after the lot has been sorted for non-conformance. The coating thickness of the repaired area must match the coating thickness of the surrounding area. However, if zinc-rich paint is used for repair, the coating thickness must be 50% higher than the surrounding area, but not greater than 4.0 mils because mud cracking tends to result when the paint coating is too thick. The maximum sizes for allowable areas that can be repaired during in-plant production are defined in the specifications as summarized below.

Maximum Size of Repairable Area

ASTM A 123/A 123M:

One inch or less in narrowest dimension

Total area can be no more than 0.5% of the accessible surface area to be coated or 36 square inches per piece, whichever is less

ASTM A 153/A 153M:

The bare spots shall have an area totaling no more than 1% of the total surface area to be coated, excluding threaded areas of the piece

ASTM A 767/A 767M:

No area given

If the coating fails to meet the requirement for finish and adherence, the bar may be stripped, regalvanized, and resubmitted

Damage done to the coating due to fabrication or handling shall be repaired with a zinc-rich formulation

Sheared ends shall be coated with a zinc-rich formulation

Repair Methods

Any repairs made to galvanized products must follow the requirements of ASTM A 780, which defines the acceptable materials and the required procedures. Repairs are normally completed by the galvanizer before the products are delivered, but under certain circumstances, the purchaser may perform the repairs on their own. The touch-up and repair materials are formulated to deliver an excellent color that matches either brightly coated, newly galvanized products or matte gray, aged galvanized products. Materials used to repair hot-dip galvanized products include zinc-based solder, zinc-rich paint, and zinc spray metallizing, and are explained in the following sections.

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Zinc-Based Solder

Figure 55: Zinc-Based SolderSoldering with zinc-based alloys is achieved by applying zinc alloy in either a stick or powder form. The area being repaired needs to be preheated to approximately 600 F (315 C). The most commonly used solders for repair, as seen in Figure 55; include zinc-tin-lead, zinc-cadmium, and zinc-tin-copper alloys.

Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be wire brushed, lightly ground, or mildly blast cleaned. In addition, if wire brushing or light blasting is inadequate, all weld flux and spatter must be removed by mechanical methods. The cleaned area also needs be preheated to 600 F (315 C) and wire brushed while heated. Pre-flux may also be necessary to provide chemical cleaning of the bare spot. Finally, special care should be given to insure that the surrounding galvanized coating is not overheated and burned by the preheating.

Application

The soldering method is the most difficult of the three repair methods to complete. A high level of caution must be taken while heating the bare spot to prevent oxidizing the exposed steel or damaging the surrounding galvanized coating. Solders are typically not economically suited for touch-up of large areas because of the time involved in the process and because heating of a large surface area to the same temperature is very difficult. When the repair has been completed, the flux residue needs to be removed by rinsing the surface with water or wiping with a damp cloth.

Final Repaired Product

The final coating thickness for this repair shall be agreed upon between the galvanizer and the purchaser, and is generally in the 1 to 2 mil range. The thickness shall be measured by any of the methods in ASTM A 123/A 123M that are non-destructive. Zinc-based solder products closely match the surrounding zinc and blend in well with the existing coating appearance.

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Zinc-Rich Paint

Figure 56: Zinc-Rich PaintZinc-rich paint is applied to a clean, dry steel surface by either a brush or spray as seen in Figure 56, and usually contains an organic binder pre-mix. Zinc-rich paints must contain either between 65% to 69% metallic zinc by weight or greater than 92% metallic zinc by weight in dry film. Paints containing zinc dust are classified as organic or inorganic, depending on the binder they contain. Inorganic binders are particularly suitable for paints applied in touch-up applications around and over undamaged hot-dip galvanized areas.

Surface Preparation

According to ASTM A 780, the surface to be repaired shall be blast cleaned to SSPC-SP10/NACE No.2 near white metal for immersion applications and SSPC-SP11 near bare metal for less aggressive field conditions. When blasting or power tool cleaning is not practical, hand tools may be used to clean areas to be reconditioned. The blast cleaning must extend into the surrounding, undamaged, galvanized coating.

Application

This method of repairing galvanized surfaces must take place as soon as possible after preparation is completed and prior to the development of any visible oxides. The spraying or brushing should be in an application of multiple passes and must follow the paint manufacturers specific written instructions. In addition, proper curing of the repaired area must occur before the product is put through the final inspection process. This repair can be done either in the galvanizing plant or on the job site and is the easiest repair method to apply because limited equipment is required. Zinc-rich painting should be avoided if high humidity and/or low temperature conditions exist because adhesion may be adversely affected.


The coating thickness for the paint must be 50% higher than the surrounding coating thickness, but not greater than 4.0 mils, and measurements should be taken with either a magnetic, electromagnetic or eddy current gauge. Finally, the surface of the painted coating on the repaired area should be free of lumps, coarse areas, and loose particles.

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Zinc Spray Metallizing

Figure 57: Zinc Spray MetallizingZinc spray, which is also referred to as metallizing, is done by melting zinc powder or zinc wire in a flame or electric arc and projecting the liquid zinc droplets by air or gas onto the surface to be coated, as seen in Figure 57. The zinc used is nominally 99.5% pure or better and the corrosion resistance of the wire or powder is approximately equal.

Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be blast cleaned to SSPC-SP5/NACE No.1 near white metal and must be free of oil, grease, weld flux residue, weld spatter and corrosion products. The blast cleaning must extend into the surrounding, undamaged, galvanized coating.

Application

Zinc spraying of the clean, dry surface must be completed by skilled workers and should take place within four hours after preparation or prior to development of visible oxides. Spraying should also be done in horizontal overlapping lines, which yield a uniform thickness more consistent than the crosshatch technique. The zinc coating can be sealed with a thin coating of low viscosity polyurethane, epoxy-phenolic, epoxy, or vinyl resin. The details of the application sequence and procedures can be found in ANSI/AWS C2.18-93. The application of zinc spray can be done either in the galvanizers plant or at the job site. In addition, if high humidity conditions exist during spraying, adhesion may be degraded.


The renovated area shall have a zinc coating thickness at least as thick as that specified in ASTM A 123/A 123M for the thickness grade required for the appropriate material category. These thickness measurements should be taken with either a magnetic or an electromagnetic gauge for best results. The plain zinc sprays or the sprays with aluminum additives both provide a good match for newly galvanized, bright surfaces. Finally, the surface of the sprayed zinc coating should be free of any lumps, coarse areas, and loose particles.

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Course Outline

Introduction

Galvanizing Process

Surface Preparation

Galvanizing

Time to First Maintenance

Other Corrosion Protection Systems


ASTM A 123/ A 123M

ASTM A 153/A 153M

ASTM A 767/A 767M

Other Galvanizing Standards

Types of Inspection

Coating Thickness

Coating Weight

Finish & Appearance

Different Appearances

Finish: Visual Defects

Additional Tests

Sampling

Repair

Zinc-Based Solder

Zinc-Rich Paint

Zinc Spray Metallizing

Test

Instructions

This course is intended to train individuals on the proper inspection techniques and requirements for hot-dip galvanized steel products. The course is expected to take 2-4 hours, so it should be taken when you can devote that amount of time to learning. There are four sections in this course:

Hot-Dip Galvanizing Process


Types of Inspection

Repair

To complete course:

Work through the course from start to finish (Introduction to Repair)

Take the Inspection Course Test by clicking the Test link

Upon successful completion of the test (80% or better), you will receive a Certificate of Completion you can print or save for your records, and you will be listed on the American Galvanizers Associations website as an inspector of hot-dip galvanized steel

If the test is not completed successfully, you will have the opportunity to retake the test 2 more times. After 3 tries, if the test is not completed successfully, you will have to retake the course.

Frequently Asked Questions

The following is a list of frequently asked questions about hot-dip galvanizing. Click on the question to be taken to the answer listed further down on the page. If you do not see your question listed here, try searching using the links above, the search engine on the top right of the page, or contact the AGA for assistance.

1. How does galvanizing protect steel from corrosion?

2. What are the steps in the galvanizing process?

3. How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?

4. How long can I expect my galvanized steel project to last in service?

5. Does the galvanized steel coating of zinc resist abrasion?

6. What causes wet storage stain and how can it be prevented?

7. Why do galvanized steel appearances differ from project to project and galvanizer to galvanizer and is there any difference in the corrosion protection offered by the different appearing coatings?

8. Can galvanized steel in service withstand high temperatures for long periods of time?

9. Why would you want to paint over galvanized steel?

10. What are the specifications governing hot-dip galvanized steel?

11. Isnt galvanizing more expensive than paint?

12. What if the article to be galvanized is larger than the dimensions of the galvanizers kettle? Can it still be galvanized?

13. What is the difference between hot-dip galvanized fasteners and zinc-plated fasteners?

14. How long will hot-dip galvanizing protect my steel from corrosion?

15. Are there any special design and fabrication considerations required to make steel ready for hot-dip galvanizing?

16. Where are galvanized steel products used?

17. What are the size limitations of steel that is to be galvanized?

18. What types of products can be galvanized?

19. Sometimes the galvanized coating is shinier in some places than others. Why is that?

20. Is the zinc coatings thickness consistent over the entire piece?

21. What can I do to minimize possible warping & distortion? Is it possible to determine prior to galvanizing which pieces might be prone to this occurrence?

22. Can I paint right over the galvanized coating? If so, what procedure should be followed?

23. How much weight will my material gain from galvanizing?

24. Are slip-critical connections a concern when the steel is to be galvanized?

25. Im interested in specifying hot-dip galvanizing for reinforcing steel. Are there any concerns with fabricating rebar after galvanizing?

26. Can I specify how much zinc to put on the steel?

27. What does it mean to double-dip steel?

28. What is the reason for incorporating venting & drainage holes into a projects design?

29. If I stitch-weld, will there be uncoated areas after galvanizing?

30. What is white rust and how can it be avoided?

31. Is there a way to provide for intentionally ungalvanized areas?

32. Is there any environmental impact when the zinc coating sacrificially corrodes? Is zinc a safe metal?

33. Should I be concerned when galvanized steel comes in contact with other metals?

34. What is the difference between hot-dip galvanizing after fabrication and continuous hot-dip galvanized sheet?

35. What is a G90 or A60 coating?

36. Is a salt spray test in a laboratory appropriate to estimate the corrosion rate of zinc coated steel?

37. Can galvanized steel in service withstand high temperatures for long periods of time?

38. Can I specify how much zinc to put on the steel?

39. What is cold galvanizing?

1. How does galvanizing protect steel from corrosion?

Zinc metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfides to attack the steel. Zinc is more importantly anodic to steel meaning it will corrode before the steel, until the zinc is entirely consumed.

2. What are the steps in the galvanizing process?

There are four steps:

1. Pre-inspection where the fabricated structural steel is viewed to ensure it has, if necessary, the proper venting and draining holes, bracing, and overall design characteristics necessary to yield a quality galvanized coating

2. Cleaning steel is immersed in a caustic solution to remove organic material such as grease and dirt, followed by dipping in an acid bath (hydrochloric or sulfuric) to remove mill scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and retards further oxidation of the steel (steel will not react with zinc unless it is perfectly clean)

3. Galvanizing the clean steel is lowered into a kettle containing 850 F molten zinc where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and one pure zinc layer

4. Final inspection the newly galvanized steel is sight-inspected (if it looks good, it is), followed up by measurement of coating thickness with a magnetic thickness gauge

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Back to Top3. How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?

When compared with paint systems, hot-dip galvanizing after fabrication has comparable initial application costs and, almost always, lower life-cycle costs. In fact, the lower life-cycle costs of a hot-dip galvanized project make galvanizing the smart choice for today and tomorrow.

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Back to Top4. How long can I expect my galvanized steel project to last in service?

Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions. To get a good idea of how long your project will last, see the service-life chart.

Back to Top5. Does the galvanized steel coating of zinc resist abrasion?

The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and have excellent abrasion resistance.

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6. What causes wet storage stain and how can it be prevented?

Zinc on newly galvanized steel is very reactive and wants to form zinc oxide and zinc hydroxide corrosion products that eventually become the stable zinc carbonate. When galvanized steel is tightly stacked or stored in wet boxes that dont allow for free flowing air, the zinc forms excessive layers of zinc hydroxide, otherwise known as wet storage stain. Most wet storage stain can be easily removed with a cleaner or nylon brush. To prevent wet storage stain, store galvanized steel indoors or block it so that there is ample free flowing air between each galvanized article.

Back to Top7. Why do galvanized steel appearances differ from project to project and galvanizer to galvanizer and is there any difference in the corrosion protection offered by the different appearing coatings?

The steel chemistry is the primary determinant of galvanized coating thickness and appearance. Continuously cast steel produced by the steel companies has a wide variety of chemistries, thus the different coating appearances. There are several different additives that galvanizers may put in their zinc kettle to enhance the coating appearance by making it shiny, spangled or matte gray. The appearance of the coating (matte gray, shiny, spangled) does nothing to change the corrosion protection of the zinc coating.

Back to Top8. Can galvanized steel in service withstand high temperatures for long periods of time?

Constant exposure to temperatures below 3900F (2000C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 3900F (2000C) on an intermittent basis.

Back to Top9. Why would you want to paint over galvanized steel?

Called duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2X the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings make for easy repainting, excell

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